Even though current technologies of fiber laser have made significant progress toward achieving a compact and reliable fiber laser system to project high quality output laser with ever increasing output energy, those of ordinary skill in the art are still confronted with technical limitations and difficulties. Specifically, the Chirped Pulse Amplification (CPA) laser system is of critical importance and often applied to generate short pulse high-energy laser pulses. In order to achieve the purpose of providing compact, reliable and high stable laser system, attempts are made to remove the free space components such as the grating lens combination for pulse chirping and de-chirping as that implemented in the conventional high-energy fiber laser systems. However, due to the small dispersion in the commonly available standard mode fibers and the relatively narrow bandwidth outputted directly from the fiber mode locking oscillator, a stretcher has to be implemented with long fiber. A fiber stretcher with longer length introduces additional problems due to a larger amount of third order dispersion. A long fiber stretcher also requires a compressor with longer length of grating pair for de-chirping, that requires large footprint of the whole system. A long fiber stretcher may further cause potential problems of system robustness, reliability and stability.
For example, a relatively narrow bandwidth directly from the fiber mode-locking oscillator, e.g., for Yb: fiber, center wavelength 1030 nm, bandwidth 8-15 nm, typically 10 nm, a conventional single mode (SM) fiber stretcher must be implemented with a long fiber in order to stretch the pulse to a few tens of pico-second (ps) or hundreds of ps. For a 1030 nm Yb fiber laser at a bandwidth of 10 nm, a fiber of 100 m fiber is employed to achieve a pulse width of 30 ps and a fiber of few hundred meter is employed to stretch the pulse width to more than 100 ps. While such a large stretching ratio is good for the control of nonlinear effect in the fiber amplifier chain, it is also leads to a serious problem due to the fact that it leaves large third-order dispersion uncompensated. The uncompensated TOD causes additional problems to the fiber laser system because the uncompensated third order dispersion (TOD) affects the compressibility of the amplified pulses. The greater the uncompensated TOD the lower the laser compressibility becomes. Actually, for current existing and demonstrated high-energy fiber laser system, the uncompensated TOD is a main issue, which makes it very difficult to achieve <200 fs high-energy pulses output.
Therefore, a need still exists in the art of fiber laser design and manufacture to provide a new and improved configuration and method to provide fiber laser to enhance the stretching ratio in the high-energy fiber laser system such that the above-discussed difficulty may be resolved.